Hydrothermal Decomposition of Cobalt Hydroxide in Saturated Water Vapor Arpit Dwivedi,* ,† Brajendra K. Sharma, ‡ Nandakishore Rajagopalan, ‡ and Sanjiv Sinha † † Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States ‡ Illinois Sustainable Technology Center, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, Illinois 61820, United States ABSTRACT: The hydrothermal decomposition of cobalt hydroxide is of importance in understanding corrosion in nuclear reactors, in the industrial production of cobaltous oxide, and potentially for thermal energy storage. The kinetics of decomposition in the presence of water vapor is poorly understood but nevertheless important in the above situations. The decomposition reaction has mainly been studied in air or inert environments. Here, we report data on the kinetics of the decomposition reaction at temperatures up to 270 °C in the presence of saturated water vapor. We show that CoO can be obtained as the decomposition product under a low dissolved oxygen level of <2 mg/L. The decomposition follows the Avrami Erofeev kinetics model with rate constants of 0.3 h -1 and 0.56 h -1 at 260 and 270 °C, respectively. In comparison, decomposition in N 2 and air environments showed much faster rates on the order of min -1 . Data reported here are important in the fundamental understanding of the reaction kinetics and in identifying the mechanism for the decomposition of cobalt hydroxide and other brucite-like hydroxides. ■ INTRODUCTION The thermodynamics and kinetics of the Co(OH) 2 to CoO reaction under hydrothermal conditions is important in numerous applications. For example, in nuclear reactors, the hydroxide and oxide of cobalt can both deposit on alloy surfaces as corrosion products. 1 Hydrothermal decomposition of Co(OH) 2 is also used to produce cobaltous oxide (CoO) that can be more homogeneous and stoichiometric compared to that produced through vacuum decomposition of Co- (OH) 2 . 2 Such CoO finds wide applications in lithium-ion batteries, 3-6 alkaline batteries, 7-9 fuel cells, 10,11 the ceramic industry, oxygen sensors, 12 and oxygen evolution reactions. 13,14 Finally, the Co(OH) 2 -CoO reaction is also promising for thermal energy storage. 15 When doped with Mg, the reaction shows high energy density and reversibility for thermal storage in the intermediate temperature range (∼280 °C). Ryu et al. 15 reported the dehydration of Co(OH) 2 at 280 °C and the hydration of CoO at 110 °C under 57.8 kPa for heat storage application. The kinetics of hydration and dehydration reactions determine the rate at which the energy can be stored and released. In particular, it is important to study the reaction kinetics under various pressure (P)-temperature (T) conditions. In a closed system, hydrothermal conditions provide a simple means of controlling water vapor pressure at different temperatures. The decomposition behavior of cobalt and other transition metal hydroxides have been extensively studied in air, vacuum, and inert environments. In vacuum (P < 10 -5 Torr), Co(OH) 2 decomposes to CoO at ∼140 °C and CoO remains the most stable phase above 400 °C. 16 In air, Co(OH) 2 decomposes to Co 3 O 4 above 350 °C which then coverts to CoO at T > 800 °C. 17-20 In an inert environment, Co(OH) 2 decomposes to CoO at T > 180 °C. 21,22 There is no general agreement in the literature regarding the decomposition mechanism of brucite- like hydroxides. Decomposition in an inert environment has been variously reported to follow the contracting geome- try, 23,24 the first order, 25 and the Avrami-Erofeev 26 model. However, very limited data have been reported on the hydrothermal decomposition of Co(OH) 2 and other transition metal hydroxides. Pistorius 27 studied Co(OH) 2 decomposition at water pressures up to 100 kbar. At 80 kbar, Co(OH) 2 decomposes into CoO at T > 320 °C. Ziemniak et al. 1 and Basavalingu et al., 2 studied stability and thermodynamics of the CoO-H 2 O system under hydrothermal conditions and reported the P-T stability curve for the system. Swaddle et al. 28 studied hydrothermal decomposition of Ni(OH) 2 in alkaline hydrothermal conditions under nitrogen, where NiO was the final product. Hazell et al. 23 studied hydrothermal decomposition and reported the kinetics to be much slower Received: October 4, 2019 Revised: December 11, 2019 Accepted: December 23, 2019 Published: December 23, 2019 Research Note pubs.acs.org/IECR Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.iecr.9b05478 Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX Downloaded via BIU SANTE on December 31, 2019 at 07:27:44 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.